Image conversion method and device for integration of a security feature into a digital image

Abstract

An image conversion method is provided for integrating a security feature in a digital source image to generate a target image secured by the integrated security feature. The image conversion method includes carrying out transverse-wave-shaped distortion of the source image to generate an intermediate image; generating a target image, the intermediate image being scanned by rows of pixels to define, per row of pixels of the intermediate image, a sequence of successive pixels according to the scanning, and each pixel of the sequence being transformed to a corresponding pixel of the target image. The arrangement of the respective pixels of each sequence is a transverse-wave-shaped wave packet in the target image; and the integrated security feature is defined by the wave packets.

What is claimed is: 
     
         1 . An image conversion method ( 200 ) for integration of a security feature into a digital source image ( 305 ) in order to generate a target image ( 455 ) secured by the integrated security feature, wherein the image conversion method ( 200 ) comprises:
 acquiring ( 205 ) source image data which represent a digital source image ( 305 ) to be protected by means of the security feature, which digital source image comprises pixels ( 315 ) arranged in a grid of straight parallel rows of pixels ( 310 ), each having at least one pixel value per pixel ( 315 );   generating ( 210 ) intermediate image data, which represent an intermediate image ( 400 ) that results from the source image ( 305 ) by applying a distortion rule, according to which, for each row of pixels ( 310 ) of the source image ( 305 ), the respective pixel values of pixels ( 315 ) of the row of pixels ( 310 ) are transferred within the grid along a direction which is angled to the row of pixels ( 310 ), in particular perpendicular to the row of pixels ( 310 ), to a respective other pixel ( 325 ;  500 . of the grid which, in relation to the respective pixel ( 315 ), is determined or determinable by the distortion rule in such a way that the arrangement ( 405 ;  410 ;  415 . of these other pixels ( 325 ;  500 ) in the grid has a transverse waveform; generating ( 220 ) target image data representing the target image ( 455 ), wherein:
 the intermediate image ( 400 ) is scanned in rows of pixels in order to define for each row of pixels of the intermediate image ( 400 ) a sequence of pixels ( 425 ,  430 ,  435 ,  440 ,  445 ,  450 ) that follow each other according to the scanning, the pixel values of which have resulted from the transfer of corresponding pixel values from the source image ( 305 ) in accordance with the distortion rule; 
 each pixel ( 425 ,  430 ,  435 ,  440 ,  445 ,  450 ) of the sequence is transformed into a respective corresponding pixel ( 425   a ,  430   a ,  435   a ,  440   a ,  445   a ,  450   a ) of the target image ( 455 ) by determining its position in the target image ( 455 ) based on its position in the intermediate image ( 400 ) by compensating for the distortion suffered by applying the distortion rule when generating the intermediate image ( 400 ), so that the arrangement ( 405 ;  410 ;  415 ) of the respective pixels of each sequence in the target image ( 455 ) represents a transverse-wave-shaped wave packet; and 
 the integrated security feature is defined by the wave packets. 
   
     
     
         2 . The image conversion method ( 200 ) according to  claim 1 , further including:
 generating ( 225 ) a physical reproduced image ( 165 ;  465 ) of the target image ( 455 ) on a surface of a substrate ( 160 ), wherein the reproduced image ( 165 ;  465 ) of the target image ( 455 ) is generated by serially generating pixels on the substrate ( 160 ) by, in order to generate a series of pixels on the substrate ( 160 ), which corresponds to a respective sequence of pixels of the target image ( 455 ), generating these pixels of the series on the substrate ( 160 ) in accordance with the pixel order defined by the sequence of the corresponding pixels of the target image ( 455 ).   
     
     
         3 . The image conversion method ( 200 ) according to  claim 2 , wherein the physical reproduced image ( 165 ;  465 ) of the target image ( 455 ) is generated on the substrate ( 160 ) using laser inscription, in which the pixels of the reproduced image ( 165 ;  465 ) are sequentially generated on the substrate ( 160 ) using a laser beam ( 130 ). 
     
     
         4 . The image conversion method ( 200 ) according to  claim 3 , wherein the different positions of the series at which the laser beam ( 130 ) strikes the substrate ( 160 ) to generate the pixels of the reproduced image ( 165 ;  465 ) are controlled by variable deflection of the laser beam ( 130 ) in a mirror-based laser galvanometer ( 120 ). 
     
     
         5 . The image conversion method ( 200 ) according to  claim 4 , wherein at least one mirror ( 135 ;  145 ) of the laser galvanometer ( 120 ) is controlled using a control signal for controlling a respective position of a pixel of the reproduced image ( 165 ;  465 ) to be generated on the substrate ( 160 ), which control signal is defined as a function of the compensation that has been determined for that pixel of the intermediate image ( 400 ) which corresponds to the pixel of the reproduced image ( 165 ;  465 ) to be generated. 
     
     
         6 . The image conversion method ( 200 ) according to  claim 4 or 5 , wherein a laser galvanometer ( 120 ) is used for variable deflection of the laser beam ( 130 ), in which the inertia of at least one of its mirrors ( 135 ;  145 ) used for deflection is so large that, when imaging the wave packets of the target image ( 455 ) by generating the pixels of the reproduced image ( 165 ;  465 ) on the substrate ( 160 ), deviations between the reproduced image ( 165 ;  465 ) and the target image ( 455 ) arise in the event of abrupt changes in direction along the course of the wave packets. 
     
     
         7 . The image conversion method ( 200 ) according to  any one of the preceding claims , wherein the arrangement ( 405 ;  410 ;  415 ) of the respective pixels of each sequence in the target image ( 455 ) is determined so that it represents a transverse-wave-shaped wave packet ( 460 ) which is periodic at least in some portions. 
     
     
         8 . The image conversion method ( 200 ) according to  claim 7 , wherein the arrangement ( 405 ;  410 ;  415 ) of the respective pixels of each sequence in the target image ( 455 ) is determined so that it represents a transverse-wave-shaped wave packet ( 460 ) which is sinusoidal at least in some portions. 
     
     
         9 . The image conversion method ( 200 ) according to  any one of the preceding claims , wherein the arrangement ( 405 ;  410 ;  415 ) of the respective pixels of each sequence in the target image ( 455 ) is determined such that each of two adjacent wave packets are separated from each other by a gap. 
     
     
         10 . The image conversion method ( 200 ) according to  claim 9 , wherein the substrate ( 160 ) is selected or processed in such a way that it has a color that stands out relative to the average of the colors of the pixels that form the wave packets of the target image ( 455 ), which are determined according to the pixel values, in such a way that neighboring wave packets can be visually distinguished. 
     
     
         11 . The image conversion method ( 200 ) according to  any one of the preceding claims , wherein each of the wave packets resulting in a respective sequence has at least two inflection points. 
     
     
         12 . The image conversion method ( 200 ) according to  any one of the preceding claims , wherein the grid of pixels of the source image ( 305 ) has rows and columns, and the resolution of the rows is different from the resolution of the columns. 
     
     
         13 . The image conversion method ( 200 ) according to  claim 12 , wherein the resolution of the source image ( 305 ) in the direction orthogonal to the rows of pixels is at most 70% of the resolution in the direction running along the rows of pixels. 
     
     
         14 . The image conversion method ( 200 ) according to  claim 12 or 13 , wherein the resolution of the source image ( 305 ) in the direction orthogonal to the rows of pixels is at least 200 pixels per inch or per 2.54 cm, PPI. 
     
     
         15 . The image conversion method ( 200 ) according to  any one of the preceding claims , wherein the pixels of the target image ( 455 ) are determined in such a way that their respective extents are the same along and orthogonal to the transverse direction of the wave packets. 
     
     
         16 . The image conversion method ( 200 ) according to  any one of the preceding claims , wherein the distortion rule is defined such that when it is applied to rows of pixels of the source image ( 305 ), at least for a subset of the rows of pixels, the wave packet respectively resulting from this has a first waveform in one or more portions of its course and a second waveform different from the first waveform in at least another portion of its course. 
     
     
         17 . An image conversion device ( 105 ) which is configured to carry out the image conversion method ( 200 ) according to any one of  claims 1 to 16 . 
     
     
         18 . A computer program or computer program product, comprising instructions which cause the image conversion device ( 105 ) according to  claim 17  to carry out the image conversion method ( 200 ) according to any one of  claims 1 to 16 . 
     
     
         19 . A digital image ( 700 ) with an integrated security feature, obtainable by the image conversion method ( 200 ) according to any one of  claims 1 to 16  as a target image ( 455 ) or reproduced image ( 165 ;  465 ) of the same on a substrate. 
     
     
         20 . A digital image ( 700 ) with an integrated security feature, in particular according to  claim 19 , having a plurality of mutually parallel rows of pixels, each of which has a transverse-wave-shaped course, wherein adjacent rows of pixels are separated from one another by a gap which, as regards its color, stands out at least in some portions with respect to the rows of pixels separated thereby, wherein the security feature is defined by the wave-shaped course of the rows of pixels and of the gaps between them. 
     
     
         21 . The digital image ( 700 ) with integrated security feature according to  claim 19 or 20 , wherein the digital image is formed on a document page serving as a substrate ( 160 ) for a value document or a security document.